MEMBRANE PERMEABILITY AND DRUG STRUCTURE 53 



types is the electrostatic force which may arise by the 

 fact that the colloidal constituents of the blood are unable 

 to pass the glornerular membrane, whereas the crystalloid 

 ions which neutralise the charge on the colloids are able 

 to pass the membrane. From this it follows that where a 

 bulk flow occurs, as in an ultra-filtration, all molecules 

 of the same charge type display the same permeability, 

 provided they are small compared with the diameter of 

 the pores through which the bulk flow occurs. All un- 

 charged molecules have the same permeability. All uni- 

 valent positive ions have the same permeability, but 

 differ from the uncharged molecules. All univalent neg- 

 ative ions have the same permeability, but differ from 

 the uncharged molecules and from the univalent positive 

 ions, etc. 



The second group is that in which permeation occurs 

 by thermal diffusion through membranes the pores of 

 which are large compared with the diameters of the 

 diffusing molecules. In this case the molecules diffuse 

 at different rates through the water filling the pores of 

 the membrane, whereas in the previous case all the 

 molecules were carried along by the bulk flow of the 

 fluid in which they were dissolved. When diffusing 

 through a membrane in this manner we find that the 



relationship 



PMVi = constant (6) 



is obeyed (P = permeability, M = molecular weight 

 of diffusing molecule). Tables vi, Vii and viii show the 



